1. Field of the Invention
The present invention relates to a single-component developer for developing a so-called electrostatic or magnetic latent image formed in an electrophotographic, electrostatic printing, magnetic printing or electrostatic recording process.
2. Description of the Prior Art
In the field of image forming processes there are already known various photographic, recording and printing processes comprising the step of rendering an electrostatic or magnetic latent image visible by means of fine colored particles called toner.
For example, the electrophotographic processes, as disclosed in the U.S. Pat. No. 2,297,691, Japanese Patent Publication Nos. Sho 42-23910 and Sho 43-24748, generally comprise steps of forming an electrostatic latent image by various means on a photosensitive member utilizing a photoconductive material, developing said latent image with toner, transferring, if necessary, the thus obtained toner image onto a transfer material such as a paper sheet, and fixing said toner image by heat, pressure or a solvent vapor to obtain a completed copy. For rendering an electrostatic latent image visible with toner there are already known, for example, a magnetic brush developing method as disclosed in U.S. Pat. No. 2,874,063, a cascade developing method as disclosed in U.S. Pat. No. 2,618,552, and a powder cloud developing method as disclosed in U.S. Pat. No. 2,221,776. The developers to be employed in such developing methods are classified into a either single-component developer solely composed of fine colored particles, generally called toner, adapted to be more or less selectively attracted or repelled by the electrostatic charge, or a two-component developer composed of a mixture of such toner particles with carrier particles such as iron powder or glass beads. In the latter two-component developer the insulating toner particles are charged by frictional charging caused by mixing the toner particles with the carrier particles and thus are deposited on the electrostatic image. Also in the former single-component developer the toner particles are deposited on the electrostatic image by a charge induced by a conductive material such as a magnetic metal sleeve for holding said toner particles, by a corona charging or by a frictional charging.
Among the developing methods utilizing such single-component developer consisting solely of toner particles, already known is the so-called charge induction developing method as disclosed, for example, in Japanese Patent Publication No. Sho 37-491. In this method electroconductive magnetic toner particles are adhered to a sleeve containing a magnet to form a magnetic brush which is maintained in contact with a member for carrying the electrostatic latent image thereon to develop said latent image with said toner. In such developing method, since the toner particles are electroconductive, a charge of a polarity opposite to that of the electrostatic latent image is induced in said toner particles when the magnetic brush is brought into facing relationship with the electrostatic latent image, whereby the development thereof is achieved by the electrostatic attraction between the latent image and the toner wherein the electrostatic charge is thus induced.
Such charge induction developing method is advantageous in that the developer, being solely composed of toner particles, does not require the adjustment of the toner concentration or the mixing ratio with the carrier particles, that the developing apparatus can be rendered compact and simple as the method does not require an agitating step for charging the developer, and that said method is free from deterioration of the quality of developed image resulting from deterioration of the carrier particles caused with the lapse of time. Despite such advantages, the charge induction developing method has been applied only to a process of forming an electrostatic latent image on a photosensitive paper coated with a photosensitive material such as zinc oxide and directly developing said latent image with the toner.
Although the image forming processes comprising a step of transferring the developed image onto a suitable transfer material such as a plain paper sheet have been widely used in recent years, the application of the above-mentioned charge induction developing method to such image forming processes is extremely difficult and disadvantageous, as the transfer step is subjected to various limitations due to the electrical conductivity of the toner particles.
More specifically, for example, in an electrostatic transfer method such as a corona transfer method in which a transfer material is maintained in contact with the member carrying the electrostatic latent image and a corona discharge is applied from the back side of said transfer material to achieve the transfer of image, the electroconductive toner particles are charged to the same polarity as that of the transfer material by a weak corona current penetrating the transfer material, whereby not only is the transfer achieved insufficiently but also the mutual repulsion between the toner particles in the toner layer thus charged to the same polarity leads to a distortion of the developed image.
Also among the developing methods utilizing a single-component developer there is already known a method utilizing insulating toner, in which toner particles are absorbed or applied on a surface of a sheet or a roller (hereinafter called "toner supporting surface") to form a toner layer which is brought into a close facing relationship or into contact with a surface carrying an electrostatic latent image thereon, thereby achieving the development. In this method the toner particles are charged either by a corona discharge or by frictional charging between the toner particles and the toner supporting surface or a surface of the developing apparatus. It is, however, difficult to achieve a uniform charging of the toner particles by a corona discharge while the frictional charging is hardly capable of providing a sufficient and stable charging as the frequency of contact of the toner particles with the surface of apparatus is much less than the frequency of contact of the toner particles with the carrier particles in a two-component developer. Although the charging of toner particles can be intensified to a certain extent by coating the toner supporting surface with a material to be charged to the opposite polarity upon friction contact with the toner particles, such measure is defective since toner particles are gradually deposited on the toner supporting surface or other surfaces by physical adhesion other than the electrostatic attraction to hinder frictional charging and also since such material is gradually abraded to lose the charging ability.